1. Field of the Invention
The present invention relates to supertwisted nematic liquid crystal display devices, and more particularly to supertwisted nematic liquid crystal display devices suited to liquid crystal display of which a high contrast ratio and high-speed responses are required.
2. Description of the Related Art
As typically shown in FIG. 1, conventional nematic liquid crystal display devices are fabricated by forming patterns of transparent electrodes 3, 4 on the lower surface of an upper glass substrate 1 and on the upper surface of a lower glass substrate 2, respectively, from indium oxide, forming a film, for example, of a polyimidosilane high polymer over each electrode and rubbing the surface of the film with a cloth in a given direction to form orientation films 5, 6 and obtain a liquid crystal cell, enclosing a liquid crystal composition in the cell to form a liquid crystal layer 7, and arranging polarizers 8, 9 respectively on the upper side and the lower side of the resulting assembly. The relationship between the rubbing angle and the twist angle of liquid crystal molecules in the liquid crystal display device is such that as seen in FIG. 2, the angle made by the rubbing direction 10 of the upper orientation film 5 and the rubbing direction 11 of the lower orientation film 6 is the same as the twist angle .phi. of liquid crystal molecules. The electrooptical characteristics of the liquid crystal display device can be represented, for example, by a voltage-transmittance curve as shown in FIG. 3.
Such a liquid crystal display device of the matrix type is usually driven optimally by the voltage averaging method with N in the number of multiplex driving. At this time, the ratio .alpha. of the effective voltage Vrma(ON) on lighted pixels to the effective voltage Vrma(OFF) on pixels which are off is expressed by: ##EQU1##
In the above expression, if N.fwdarw..infin., .alpha..fwdarw.1. Thus, with an increase in the number N, the difference between Vrma(OFF) and Vrma(ON) decreases. Accordingly, in realizing matrix-type liquid display devices for displaying an increased amount of data, it is required to develop liquid crystal materials having sharp threshold characteristics and improved orientation films in order to obtain a satisfactory contrast ratio even if the voltage difference between Vrma(ON) and Vrma(OFF) is small. In the field of liquid crystal display devices, there is an increasing demand for greater display capacities especially in recent years with an increase in the amount of data to be displayed.
In corresponding relation with the increase in the number N, there is recently a tendency to set the twist angle .phi., which is conventionally 90.degree., to as large as 210.degree. to 270.degree..
For liquid crystal display devices wherein the liquid crystal molecules have such a great twist angle .phi., nematic liquid crystal compositions are used which have incorporated therein an increased amount of optically active substance for inducing a twisted helical structure of liquid crystal molecules. The amount of optically active substance to be used is so adjusted that the ratio of the thickness d of the liquid crystal layer to the resulting helical pitch p of the nematic liquid crystal composition, i.e., d/p, is in the range of .phi./360-0.25.ltoreq.d/p.ltoreq..phi./360+0.25. Incidentally, as the twist angle .phi. is increased, the contrast ratio increases (see FIG. 4), hence desirable, whereas the range of d/p values settable without entailing improper orientation (undertwist domain or stripe domain) then tends to become smaller (see FIG. 5).
Especially if the range of d/p values settable decreases to not greater than 0.1, problems will arise in respect of reliability, etc. Attempts have therefore been made to increase the range of settable d/p values by adding an increased amount (e.g., at least 20 wt. %) of components with a strong stripe domain inhibitory effect to liquid crystal compositions for use in supertwisted nematic liquid crystal display devices wherein the liquid crystal molecules have a twist angle of at least 210.degree. (Unexamined Japanese Patent Publications SHO 64-56415 and SHO 64-33522, etc.)
The steepness of electrooptical characteristics of liquid crystal display devices (see, for example, FIG. 3) differs with the cell construction and the liquid crystal composition used. If the steepness is insufficient, the contrast ratio (ratio of transmittance during ON-time to transmittance during OFF-time) is low to render the display difficult to view, whereas excessively steep characteristics give rise to a problem as to the nonuniformity of the display (especially halftone display) due to variations in the cell thickness in the display device. For this reason, the steepness of electrooptical characteristics of a particular liquid crystal display device must be set to a suitable range inherent in the cell structure of that device. For example, in the case of the liquid crystal display device having the cell structure shown in FIGS. 1 and 2 and usually exhibiting the electrooptical characteristics of FIG. 3, it has been found desirable to set the steepness of the characteristics to the range of 1.04.ltoreq.V.sub.90% /V.sub.10% .ltoreq.1.08.
Recently, there is a tendency to use supertwisted nematic liquid crystal display devices in word processors, lap-top personal computers, family computers, and the like. Especially with personal and family computers which are adapted for scrolling or playing games, the display device to be used must have a high response speed for quick changes of screen images. Nevertheless, conventional supertwisted nematic liquid display devices are low in response speed and therefore still remain to be improved for use in personal or family computers. Moreover, as the contrast ratio is increased to give more distinct display, the response speed generally becomes still lower.
It appears possible to increase the response speed of liquid crystal display devices by reducing the thickness of the liquid crystal layer (cell thickness), but a reduction in the thickness of the liquid crystal layer results in a lower yield, for example, due to the dust problem involved in the process to entail an increased cost, hence undesirable.
The present invention, which has been accomplished to overcome the foregoing problems, provides a supertwisted nematic liquid crystal display device having a high response speed and adapted to give distinct display without entailing a reduced yield (increased cost).
The compounds of the formulae (I) to (VII) for use in the present invention are known as components of liquid crystal compositions. However, no specific liquid crystal composition is known which is prepared from the combination of such components.